Idling higher than normal

[Tom R]

Hi all

Hope someone can give some pointers on an issue I had the other day. I took out my 1967 MGB GT on Sunday for a run, started all fine and running really well, but after about an hour and a half of it being run I was driving through the town and it was idling at about 2,000 rpm when sat in neutral at some traffic lights. Normally idles a lot lower when it first warms up.

This higher rev may have started a little earlier in the run, say an hour in when I noticed trying to put it in first at a stop it was crunching into gear with the higher revs. I made sure the choke was all the way in so it couldn’t have been that affecting it, temps all looked okay too.

Just wondered if anyone has ever come across this issue before and have any tips on what I need to check over? Only recently been taking it out since taking ownership from my grandad so bit of a novice on it at the moment. It’s also it first proper run out of over an hour or two since it had been recommissioned with fresh spark plugs and bits by our local garage that deals with classics

Appreciate any assistance at all.

Thanks!

[Bourne]

That’s not the way round it normally works

Others will eventually pitch in who have better technical skills than I. I did spend the last summer though running down a slight drop in Revs so I understand the annoyance and pain you are about to go through.

First things first. Have you started or driven the car since you clocked the high revs? It will be useful to know if this is a problem obvious from the start, or something that kicks in once the engine has warmed up? Also where have you / your grandfather set idle. It’s normally about 700-800 rpm but some people do set it higher

Things that might cause an engine to start idling at higher revs (my starters for ten).

- Choke left engaged - that could be either though not setting it back, or the cable / lift mechanism jamming.

- Vacuum Advance. This is normally a fail of either the tube or the mechanism causing the revs to drop (the cause of my wows last year). I can’t conceive of a scenario where it miraculously starts working, but maybe you’ve changed over the pipe after your grandfather adjusted the timing to compensate for a nonworking vacuum? That leads meetly onto onto the next potential source.

- Ignition timing: Done by turning the distributer. If it’s not fully secured to the engine and someone was doing some maintenance - it could have been knocked.

- Carburettor pots: have you topped them up at all? I can’t see how that would make a 1000 Rev difference but still?

- Carburettor pins: No direct experience of this, but I’d expect a failure here to do more than just affect revs. Your experience might be a very early indicator of a problem?

- Fuel quality? When did you fill up the car. Do you use particular petrol brands. If your last tank was full of poor quality petrol and you put in premium, high octane, low ethanol petrol and than used some additives, it might account for the change (I’m not convinced of that, but again no first hand experience)

- Fuel filter. Did you swap out a particularly old and clogged up one?

Keep us updated on how you get on, and tell us how it ends. Too many technical question threads here end up with us never finding out what the culprit was,

Good hunting!

[Tom R]

Hi David

Thanks for the response, this is the third time I’ve driven it since taking ownership, first two occasions were shorter journeys of 15 mins or so to get it moving. It had been at a garage in March to get it back running where they said all it need was new spark plugs, oil filter and the points/condenser tuning up. Runs fine but just that higher idle after it’s properly warmed up (30mins into a run).

Couple of those bits raised I know answers to:

- Choke - worked fine for when I started it up cold at the beginning of the drive, made sure it was all the way in before setting off and at that point it was idling at 700/800rpm. Will check if there was an issue with the cable that may have caused that to jam.

- Fuel quality - was near empty at time of leaving the garage in March and have only filled it since with E5.

Will have to have a look at those other bits mentioned when next with the car. There was a couple moments when driving along in it after an hour when going up hill in say 3rd while higher revving would release a small quick puff of smoke like it was clearing something, couple of family members noticed this but they didn’t think it looked anything bad more like I say just clearing out. This was around the time that trying to get it in first at a complete standstill would crunch into it with the non synchro and the higher idle at that point. The idle at the point of the car warming up in the first 5 mins is at the normal 700/800 rpm, it’s just after a good 30 mins plus of driving when it was idling at that higher rpm.

I think it’s likely either the idle needs adjusting for when it has warmed up now it’s getting used more (grandad wouldn’t have gone far at all in last few occasions of him using it), issue with the choke or vacuum leak as you suspect. Will make sure to put a response in here once we find the reason, here’s hoping it just needs the idle adjusting!

Cheers!

[Pjones]

Hi.
I would suspect a vacuum leak is presenting itself as the engine has warmed up. The usual areas are the inlet manifold ( check the tightness of the securing studs/ nuts) or the carburetor butterfly spindles. You could try whilst the engine is running on tickover and showing the symptoms, if you have some carburetor cleaner or a gas blow torch spray cleaner or blow gas around the manifold joint and carb spindles. If there is a vacuum leak the rpm should rise more.
Good luck.
Paul

[Not_Anumber]

+1 for vacuum leak being very likely. Check all vacuum hoses including the large one to the brake servo if fitted plus check none of the inlet manifold and carb bolts are loose